Specific Features of Changes in the Properties of One- and Two-Alkali Borate Glasses Containing Water: I. Viscosity, Thermal Expansion, and Kinetics of Structural Relaxation in Binary Alkali Borate Glasses

Alkali borate glasses with different contents of residual water are prepared by varying the synthesis conditions. The temperature dependences of the viscosity and thermal expansion of glasses are obtained. The structural relaxation parameters are calculated from the hysteresis dilatometric curves measured. The water content is determined using the IR absorption spectra in the range of stretching vibrations of hydroxyl groups at room temperature. It is found that an increase in the water concentration in alkali borate glasses leads to a decrease in the viscosity. The character of variations in the viscosity logarithm with a change in the water content depends on the alkali cation concentration. The glass transition temperatures determined from the dilatometric curves for all the studied glasses decrease with an increase in the water content. As the water concentration increases, the thermal expansion coefficient (above and below the glass transition range) and the degree of fragility decrease for glasses containing 25 mol % Na₂O, increase for glasses with an alkali oxide content of 15 mol %, and remain virtually unchanged for glasses involving 5.5 mol % Na₂O. A change in the water content in the concentration range under investigation does not affect the structural relaxation parameters.     

IR spectroscopic study of water in the structure of binary alkali borate glasses

The influence of the synthesis conditions and composition of binary sodium and potassium borate glasses on the spectral absorption in the wavelength range 2.5–4 µm due to the presence of structurally bound water is investigated. It is demonstrated that water in the structure of alkali borate glasses, as in the structure of alkali silicate glasses, can exist in the form of so-called free and bound hydroxyl groups that are incorporated into the glass network in different ways. The ratio between the numbers of these forms of structural water in borate glasses is determined by the synthesis conditions and composition of the glass. The synthesis under different conditions makes it possible to prepare glasses not only with different water contents but also with different ratios between the numbers of free and bound hydroxyl groups.Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Golubeva, Pavinich.     

Influence of Residual Water in a Glass on Its Rheological and Relaxation Properties (by the Example of a 5Na2O · 95B2O3Glass)

Glasses of the 5Na₂O · 95B₂O₃(mol %) composition synthesized at a temperature of 1100°C for 180 and 20 min are studied. The temperature dependences of the viscosity and the thermal expansion of glasses are obtained. The thermal expansion coefficients and glass transition temperatures of the studied glasses are determined, and the parameters of structural relaxation (the constant characterizing the width of the spectrum of relaxation times, the relaxation modulus equal to the ratio of the viscosity to the relaxation time, and the relaxation time at zero reciprocal temperature) are calculated from the dilatometric curves measured at temperatures close to the glass transition range. The water content in the studied glasses is estimated by comparing the obtained dependence of the viscosity on the water content with the data available in the literature for glasses of a similar composition. The assumption is made that the structural relaxation time in sodium borate glass decreases with an increase in the water content.     

Low-Melting Glasses Based on Borate Systems

The results of analyzing systems of low-melting glasses and their application areas based on published data are used to select an optimum system. The melting properties of low-melting glasses based on the borate system ZnO – SrO – B₂O₃ are studied. The dependences of the main properties of glasses (TCLE, softening temperature, microhardness, chemical resistance to water) on their compositions are determined. The optimum low-melting glass compositions with required physicochemical and technological parameters are identified.     

Specific Features of Changes in the Properties of One- and Two-Alkali Borate Glasses Containing Water: III. Thermal Expansion and the Structural and Mechanical Relaxation Parameters of Two-Alkali Borate Glasses

The thermal expansion and stress relaxation in mixed alkali borate glasses containing lithium, sodium, and potassium oxides with a total alkali oxide content of 15 mol % are measured on an inclined quartz dilatometer and a relaxometer. The experimental data obtained are used to determine the thermal expansion coefficients and the structural and mechanical relaxation parameters. No deviations from the additivity are found in the concentration dependences of the thermal expansion coefficient and the calculated parameters determining the width of the spectra of the structural and stress relaxation times. The IR absorption spectra of the studied glasses are recorded in the range of stretching vibrations of hydroxyl groups. Analysis of the IR spectra makes it possible to assume that the content of residual water in the structure of borate glasses affects the manifestation of the mixed alkali effect in the properties of these glasses.     

Effect of BaF2 Content on Luminescence of Rare‐Earth Ions in Borate and Germanate Glasses

Rare‐earth‐doped oxyfluoride germanate and borate glasses were synthesized and next studied using spectroscopic methods. Influence of fluoride modifier on luminescence properties of rare earths in different glass hosts was examined. The excitation and emission spectra of Pr³⁺ and Er³⁺ ions in the studied glasses were registered. The emission spectra of Pr³⁺ ions in germanate and borate glasses are quite different and depend strongly on the glass host. In samples doped with Er³⁺ ions emission bands located around 1530 nm corresponding to the main ⁴I_13/2→⁴I_15/2 laser transition were registered, independently of the glass host. Quite long‐lived near‐infrared luminescence of Er³⁺ ions was observed for germanate glasses with low BaF₂ content, while in borate glass systems influence of barium fluoride on luminescence lifetimes is not so evident. The Judd–Ofelt calculations were used in order to determine quantum efficiencies of excited states of rare‐earth ions in germanate and borate glasses.     

Boron-containing bioactive glasses in bone and soft tissue engineering

Boron is considered to influence the performance of several metabolic enzymes and boron deficiency is associated with impaired growth and abnormal bone development. As such, boron is a beneficial bioactive element for animals and humans. It is also well known that boron stimulates wound healing and improves bone health. The addition of boron in different proportions to bioactive glasses has significant effects on glass structure, glass processing parameters, biodegradability, biocompatibility, bioactivity and cytotoxicity. Different compositions of bioactive glasses (BGs) containing boron, including boron-doped, borosilicate and borate glasses, are being investigated for bone and soft tissue engineering under the premise that these BGs are suitable carriers of boron, indicating controlled release of B species in the biological environment. This paper reviews up to date research and applications of borate, borosilicate, and boron doped silicate and phosphate BGs focussing on their physical, structural, degradation and biological properties for hard and soft tissue regeneration.     

铝硼硅酸盐玻璃纤维结构与耐水动力学

采用红外光谱、示差扫描量热分析等手段研究了氧化硼对铝硼硅酸盐玻璃结构及转变温度的影响,并用粉末法测试了玻璃的耐水性。结果发现,随着氧化硼含量增加,玻璃转变温度先降低后升高。玻璃与水反应初期受硼酸盐的水解动力学控制,后期主要为水分子对硅氧骨架的侵蚀和Ca2 与H 离子的离子交换过程。分析认为,引入少量的B2O3主要与碱性的CaO结合,生成含有非桥氧的端氧键B-O-Ca,破坏了玻璃网络结构,B2O3过量时,更多的硼酸盐基团与硅酸盐基团结合,生成桥氧键B-O-Si键,增强了玻璃结构。     

The influence of water on the structure and glass transition of sodium borate supercooled liquids from the small-angle X-ray scattering data

Sodium borate glasses containing 5.5, 15, and 25 mol % Na₂O are synthesized under different conditions (different temperatures and times of heat treatment, treatment under vacuum) and studied by the small-angle X-ray scattering (SAXS) technique. The temperatures of the glass transition and the transition from the supercooled liquid state to the noncrystalline solid state are determined from the temperature dependences of the SAXS intensity. It is revealed that a decrease in the water content leads to an increase in the glass transition temperatures and to an increase in the SAXS intensity in the case of glasses containing 15 and 25 mol %. It is assumed that this behavior can be associated with the increase in the isothermal compressibility.     

Optical Absorption Characteristics of Ni2+ in Mixed-Alkali Borate Glasses

Nickel-containing mixed-alkali borate glasses were prepared and their spectral absorption in the visible and near-ir were measured and used to determine the ligand field strength, Racah parameter, and state of coordination of Ni²⁺ ions. The change in these parameters with alkali content is discussed in terms of mixed-alkali effect. Nickel ions were found to occupy octahedral sites in the 20 mol% (Li₂O+Na₂O) glasses, whereas in the 30 mol% (Na₂O+K₂O) glasses Ni^Z+ ions are believed to occupy both octahedral and tetrahedral sites. In most cases slight deviations were observed which increased with increasing alkali content. The results obtained suggest that the substitution of one alkali for another in alkali borate glasses did not produce gross alterations of the glass network sufficient to induce deviations from linearity in property-composition relations.     

Preparation and Properties of Nitrogen-Containing Na2O-B2O3 Glasses

Up to 3.3 wt% nitrogen can be incorporated into Na₂O-B₂O₃ glass melts. The melting procedure is described, and structure models are given. In contrast to N-containing silicate glasses, the borate glasses were transparent; however, micrographs of their fracture surfaces showed some crystallinity. Properties were determined as a function of the N and Na₂O contents of the glasses. Compared with N-containing silicate glasses, the properties of borate glasses are much less changed by the nitrogen introduced.     

The role of cadmium oxides in the enhancement of radiation shielding capacities for alkali borate glasses

The radiation shielding capacity of 80B₂O₃–(20-x) Na₂O–x CdO (where x = 0, 2, 4, 6, 8, 10, 12 and 14 mol%) cadmium-doped sodium borate glasses was evaluated. The linear attenuation coefficient (LAC) of the studied glasses was simulated using the Monte Carlo simulation code (MCNP-5) for gamma photons with energy between 0.01 and 15 MeV. Other pivotal gamma ray shielding parameters such as mass attenuation coefficient (MAC), half value layer (HVL), effective atomic number (Z_eff), and buildup factors (EBF and EABF) for the selected cadmium-doped sodium borate glasses were examined. The achieved data showed that, BNC 8 glass with CdO contents of 14 mol % possesses the highest LAC and varied between 0.0650 and 83.636 cm^-1 and also it has thinner HVL than other BNC glasses and varied between 0.008 and 10.686 cm for gamma ray energy between 0.01 and 15 MeV. Moreover, the removal cross sections of fast neutrons (Σ_R) were also calculated theoretically for the selected cadmium-doped sodium borate glasses. The obtained data shows that the insertion of CdO appears to enhance gamma ray and neutron shielding capacity. BNC8 with CdO content (14 mol%) was found to possess the best shielding parameters among the studied cadmium-doped sodium borate glasses.     

Dielectric Studies of Some Borate and Phosphate Glasses

Some borate glasses, with sag points of 350° to 500°C., of high specific resistance and low power factor have been prepared. The power factor of the glasses has been measured to 250° C., and the resistivity has been measured up to temperatures approaching the transformation region. Borate glasses have higher resistivities and power factors than phosphate glasses. An increase in sag point of lead borate glasses by substitution of PbO with BaO, Al₂O₃, or SiO₂ improves their dielectric properties. Depending on the type of glass, the dielectric constant varies linearly with the density of the glass.     

The structure of borate glass

The existing hypotheses on the structure of alkali borate glass were comprehensively analyzed. Their imperfection in explaining the dependences of the properties of the glasses on their composition was noted. Alternative schemes of structural borate complexes which more convincingly explain the characteristics of the change in the properties of this glass are proposed.     

Mössbauer Spectroscopy of Borate Glasses Containing Divalent Europium Ions

Borate glasses containing 0.99–30.0 mol% EuO have been prepared under a reducing atmosphere and ¹⁵¹Eu Mössbauer effect measurements have been carried out at room temperature in order to examine the chemical state of Eu²⁺ ions in these glasses. Mössbauer spectra indicate that most of the europium ions are present as the divalent oxidation state. In dilute Eu²⁺-containing borate glasses, spectra are split due to paramagnetic hyperfine interactions in the glasses with low and high sodium contents. In concentrated Eu²⁺-containing borate glasses, line broadening results from the contribution of quadrupole splitting due to asymmetrical oxygen coordination around Eu²⁺ and the contribution of inhomogeneous broadening due to site-to-site variation. The compositional dependence of isomer shift and quadrupole interaction parameter can be related to the structural changes in the borate glass. The variation of the isomer shift has a good correlation with the optical basicity of glass, and the trend can be explained in terms of the covalent admixture with 6 s character. The Eu-O bond in the borate glasses is more ionic than that in EuTiO₃ and EuZrO₃, where the oxygen coordination number for Eu²⁺ is 12. The average coordination number for Eu²⁺ is found to be 12 in all of the present glasses.     

Thermal Expansion and Glass Transition Temperature of Calcium Borate and Calcium Aluminoborate Glasses

The thermal expansion (the temperature coefficients of linear expansion for solid glasses and the structural temperature coefficients of linear expansion) and the glass transition temperatures are studied for glasses in the CaO–B₂O₃ and CaO–B₂O₃–Al₂O₃ systems. The results obtained are compared with the available data for barium borate and barium aluminoborate glasses. The revealed dependences are interpreted within the concepts of the borate glass structure.     

Compositional Dependence of Absorption Spectra of Ti3+ in Silicate, Borate, and Phosphate Glasses

Absorption spectra of Ti³⁺ were measured for silicate, borate, and phosphate glasses doped with 0.5 mol% Ti₂O₃.The absorption coefficient at the peak wavelength of the ₂T₂→₂E transition of Ti³⁺ is used as a parameter showing the relative content of Ti³⁺ions in glass samples. The effect of glass composition on Ti³⁺/Ti⁴⁺ redox was studied. For multicomponent glasses, a basicity parameter calculated from glass composition is proposed in terms of coulomb force between the cation and the oxygen ion. The value of the absorption coefficient depends on basicity in silicate and borate glasses; however, it is independent of composition in phosphate glasses.     

Thermal Expansion of Alkali Borate Glasses

The thermal expansion behavior of lithium, sodium, potassium, and rubidium borate glasses was measured. The results indicate that the "borate anomaly" in the thermal expansion coefficient occurs at ∼20 mol% alkali oxide. A maximum in the glass transformation and dilatometric softening temperatures also occurs at ∼27 mol% alkali oxide. No evidence for phase separation was observed in this study. These effects are related to the structural changes reported for these glasses by Bray and coworkers.     

Preparation and Properties of Silver Borate Glasses

Silver borate Basses containing 0 to 30 mol% Ag₂O were formed. Properties measured include density, thermal expansion coefficient, glass transformation and dilatometric softening temperatures, transformation-range viscosity, dc electrical conductivity, and helium permeability and diffusivity. Optical spectroscopy revealed that the color of these glasses results from absorption bands at ∼407, ∼310, and ∼250 nm. Each band increased in intensity as the silver oxide content of the glass was increased. The properties of these glasses were consistent with the structural model currently used to describe the structure of alkali borate glasses. No evidence for the existence of phase separation was found for the conditions of the present study.     

Multiple-Quantum Magic-Angle Spinning 17O NMR Studies of Borate, Borosilicate, and Boroaluminate Glasses

Multiple-quantum magic-angle spinning (MQMAS) ¹⁷O NMR spectroscopy has been applied to study boron oxide, sodium borate, sodium potassium borosilicate, and sodium boroaluminate glasses. Up to eight distinct oxygen sites are identified, with the chemical shift and quadrupolar coupling parameters determined for each. Previous assignment of the B-O-B and Si-O-B resonances has been found to be incorrect. In contrast to standard models of glass structure, three-coordinated boron mixes with the silicon oxide network to a great extent. Sodium borosilicate glasses with low-sodium content are likely to be phase-separated on the nanoscale. Those with intermediate sodium content form homogeneous glasses with boron atoms distributed evenly in the SiO₂ network. A boron avoidance rule analogous to the aluminum avoidance rule may apply in this region, when BO₄ groups are abundant. Adding excess sodium cations to the system may again lead to compositional heterogeneity. The existence of diborate group as the basic building unit in glasses with appropriate sodium content is not well supported. The network-forming cations in sodium boroaluminate glasses are well mixed, with no Al-O-Al resonance observed by MQMAS. The effect of cation type and thermal history on glass structures is also discussed.